1. Field of Invention
The present invention relates to current mode control of switching power supplies, particularly low voltage power supplies.
2. Description of Related Art
Current mode power supplies, such as the synchronous buck converter shown in FIG. 1, typically use a resistive element to sense current. This method has the drawback of causing additional circuit losses, and the sense resistor occupies space. Accordingly, it would be desirable to provide a current mode power supply which does not require a resistive element for sensing inductor current.
The present invention, in lieu of directly sensing the inductor current with a resistor, derives the inductor current by sensing the voltage drop across the synchronous MOSFET of the half-bridge and reconstructs the current using a sample and hold technique. A ripple current synthesizer is employed to reconstruct inductor current outside the sample and hold window. The sampled product ILoadxc3x97RDSon is used to update the ripple current synthesizer with dc information every switching cycle. The resulting voltage waveform is directly proportional to the inductor current.
The power converter may operate at constant switching frequency if desired. The synchronous MOSFET may be turned off after a brief sample period if desired. The inductor current synthesizer of the present invention can be used not only in a synchronous buck converter power supply, but also with boost converter, flyback converter and forward converter topologies.
Other features and advantages of the present invention will become apparent from the following description of the invention, which refers to the accompanying drawings.